The change in the beauty's expression is simply your eye playing tricks on you.
Credit: Museo del Prado
Strolling through the Louvre, you stop at Leonard da Vinci's Mona Lisa. Initially, she appears to be smiling; but as you move your gaze, the expression changes — not so happy anymore.
Among the top questions baffling art enthusiasts is the elusive grin. Did da Vinci intentionally create the ambiguous appearance?
Here's her secret: Your first stare at the legendary canvas will most likely be directly at the sitter's eyes. At this point, the part of your eye called the fovea that picks up fine details such as color will process the image of the eyes, while your imprecise peripheral vision will pick up the image of the lips. Because peripheral vision can't distinguish fine details, it mistakes the shadows from the sitter's cheekbones as a smile. When you return your gaze to the lips, your fovea sees the fine details of the lips. Voila! A smile turned upside down.